It is known to squeeze a free-flowing, pasty mass, such as a sealing compound or dental impression mass, out of a tubular bag by the latter being put into a cylinder space which is acted upon from one end by a piston and has a discharge opening or delivery nozzle at the other end, at which the bag is open. In this case, it is particularly important that that end of the tubular bag which contains the bag opening is sealed off from that end of the cylinder space and of the delivery nozzle which forms the discharge opening. If, for this purpose, the tubular bag is firmly connected to the device forming the cylinder space (FR-A 1 161 905), repeated use of the squeezing-out device for a plurality of tubular bags is made impossible. If the tubular bag is connected to the delivery nozzle in one piece or via thread (DE-A 35 00 625), the exchange of the tubular bag is prevented or made more difficult, provided the nozzle is also not designed as a throw-away part. The latter is often not possible or is undesirable. The arrangement of sealing elements between the end of the tubular bag and the delivery nozzle or discharge opening (EP-A 319 666), which sealing elements are to be fixed by special clamping or screw parts, also makes exchange difficult. To avoid these disadvantages, the arrangement mentioned in the preamble of claim 1 for squeezing out interchangeable tubular bags has been disclosed (EP-A 541 972), in which the gathered-up end, forming the bag opening, of the tubular bag is firmly connected to a sealing ring, which has a conical sealing surface, which interacts with a correspondingly conical sealing surface in the discharge opening of the squeezing-out device. Under the action of the squeezing-out piston, the cone of the sealing ring is pressed into the conical discharge opening, provided it is adequately centred beforehand. In the event of inadequate centring, sealing is not achieved. If the cone of the sealing ring sits in an exactly centred manner in the discharge opening, it appears possible to achieve an adequate sealing effect as long as the effect of the piston force continues. However, this sealing effect ceases when the arrangement "breathes" if the piston force ceases. Since, during the squeezing-out, most of the pressure drop does not take place in the bag opening or the discharge opening of the squeezing-out device but in the downstream spaces of the nozzle and the mixing device which is possibly present, a considerable pressure, which leads to corresponding extension of the associated walls, builds up during the squeezing-out in these spaces downstream of the discharge opening. If the squeezing-out force of the piston ceases, this extension leads to a back pressure and a backflow of the mass in the region of the discharge opening. As a result, the cone of the sealing ring may be lifted from its conical seat in the discharge opening; the sealing gap opens and the mass can penetrate into the space, to be sealed off, between the sealing ring and the tubular bag on the one hand and the cylinder space on the other hand. The mutual centring may also be lost, so that the cone of the sealing ring is not returned into the correct seat when pressure is next applied. Contamination of the device cannot therefore be completely avoided by the known arrangement. In addition, there is the disadvantage that the discharge opening has to be carefully cleaned when the tubular bag is changed, since otherwise possibly consolidated residues of the mass which remain there make complete sealing impossible even during the effect of the squeezing-out force, a factor which then leads to further contamination of the device. The known type of sealing also requires careful production and assembly of the parts which are involved in the sealing, and this is expensive. It is known (EP-A-663348) to counteract these disadvantages by the ring which bears the cone having a rim diameter which exceeds the inside diameter of the cylinder space accommodating the tubular bag and by said ring being fixed in a circumferential groove, which is formed between the end of the wall of the cylinder space and a lid forming the delivery nozzle. However, this has the disadvantage that the tubular bag can only be used in such devices whose cylinder space contains said circumferential groove and has a lid on the delivery side, the lid having to be opened in order to insert the tubular bag.